SOME TYPES OF PLOWBKLESS PLANTS. 



221 



lU 



268. Motile Form of Protococais. — GcoaBionally the Protococcus 

 cell may be found in an actively swimming condition, known as its 

 motile form. The larger motile cells are either naked or are covered 

 with a cell wall, which the colored cell-contents does not entirely fill. 

 The former condition is represented by Mg. 191, I, the latter by II. 

 These large motile cells may multiply by a process known as fission into 

 twos or fours, or the whole cell-contents may break up into as many as 

 32 portions, each of which then sets out 

 in an independent existence as a freely 

 swimming spore (zoospore), Fig. 191, III. 



The change from the still to the motile 

 form appears to be favored by heat, sun- 

 light, and abundance of air-supply (as 

 by shallowness of the water in which 

 the plants are growing) ; the reverse 

 change is brought about by conditions 

 just the opposite of those above men- 

 tioned. 



269. Nutrition of Protococcus. — 

 Protococcus can flourish only in the 

 sunlight, but with a sufficient supply of 

 light it can absorb and fix carbonic acid 

 gas (giving oft at the same time bubbles 

 of oxygen) and can assimilate mineral 

 substances. It is a capital example of an individual cell capable of 

 independent existence. 



Fig. 191.— MotUe Cells of Proto- 

 coccus. (Greatly magnified.) 



I, protoplasm without cell wall ; 

 II, protoplasm enclosed in a loose 

 cell wall ; III, a mucli smaller 

 motile cell (zoospore). 



THE STUDY OF SPIEOGYRA.i 



270. Occurrence. — Spirogyra, one of the plants commonly known 

 as pondscum, or "frog-spit," occurs widely distributed throughout the 

 country in ponds, springs, and clear streams. It is of ajgreen or yellowish- 

 green color, and in sunny weather usually floats on or near the surface 

 of the water buoyed up by the numerous oxygen bubbles which it sets 



' If Spirogyra is not easily found, the teacher may advantageouBly use Zygnema 

 or Mesocarpus. He should become familiar with the appearance of some of the 

 fresh-water algae by microscopical studies of them and by reference to the figures in 

 such works as Wood's Fresh -Water Algse. There are many excellent small cuts of 

 common forms in Campbell's Elements of Structural and Systematic Botany, pub- 

 lished by Ginn & Co. The teacher may consult this latter book to great advantage 

 throughout his studies on cryptogamous plants. 



